Acrylonitrile is produced commercially in systems using what is known as the "Sohio" process, described in U.S. Pat. No. 2,904,580 to Idol. The reactor feeds in a commercial acrylonitrile system using the, "Sohio" process are propylene, ammonia, and compressed air. The propylene and ammonia are vaporized, combined with the air, and fed to a fluidized bed catalytic reactor. Precise ratios of the three feeds are maintained for optimum yield.
The manufacture of acrylonitrile has four basic stages: a reaction stage, in which the ammonia and propylene are reacted; a cooling stage, in which the reaction product is cooled; an absorption stage, in which a crude acrylonitrile product is collected; and, a purification stage, in which the crude acrylonitrile product is purified.
In the reaction stage, the propylene, ammonia, and compressed air feeds are mixed together in a reactor and react on the surface of a fluidized catalyst. A set of complex exothermic reactions takes place, forming the following products: acrylonitrlle, hydrogen cyanide, carbon dioxide, carbon monoxide, acetonitrile, hydrogen acrolein, acrylic acid, water, other higher nitrltes, aldehydes, ketones, acetic acid, and a number of miscellaneous unknown organic compounds. Conversion of the three feeds is less than 100%; therefore, unreacted propylene, ammonia, oxygen, and nitrogen are contained in the reactor effluent gas.
A portion of the heat produced by the exothermic reaction is removed by sets of steam coils. Reactor effluent gas passes through cyclones, which remove catalyst fines from the gas. The gas then is cooled in a reactor effluent cooler comprised of a shell and a tube exchanger using boiler feedwater as the cooling source.
In the cooling stage, the gas leaving the reactor effluent cooler is cooled in a quench column by contact with a recirculating water stream. Most of the water vapor and small amounts of organic vapors in the gas are condensed in the quench column. The quench column bottoms are cooled and circulated back to the quench column. The excess quench water is roughly equal to the amount of water produced by the reactor and is fed to the wastewater column, where acrylonitrile and hydrogen cyanide are recovered. Wastewater column bottoms ultimately are injected into the wastewater injection well.
In the absorption stage, the quench column effluent gas is directed to an absorber where chilled water is used to absorb acrylonitrile, hydrogen cyanide, and other organics from the gas. Absorber bottoms are fed to a recovery column where a crude acrylonitrile product is taken overhead.
The crude acrylonitrile product then goes through a purification stage in a series of distillation columns. The first column (heads column) removes hydrogen cyanide, while the second column (drying column) removes water. The last column (product column) takes pure acrylonitrile monomer from a side-draw near the top of the column. Heavy ends are rejected from the product column bottoms.
Unfortunately, the acrylonitrile monomer can polymerize during the cooling stage in the quench column and during the purification stage in the distillation columns. The acrylonitrile that does polymerize in the quench column and/or distillation columns represents an undesirable net product loss for the acrylonitrile plant.
Inexpensive compounds that effectively inhibit the premature polymerization of acrylonitrile during its manufacture are sorely needed.